Prostheses have been developed to replace a wide variety of body parts. Some prostheses, in particular prosthetic body joints, include at least one element anchored to bone. When the bone anchoring element is first implanted, it is preferable that no external forces be applied to the embedded element, and therefore to the bone, until the bone has had time to heal. By giving it this time, the bone is more likely to grow and bond with the anchoring element, and thereby increase the strength of the bond between the prosthesis and the bone. In general, bone is less likely to bond to an embedded element if external forces are applied soon after implantation.
In time, one or more elements of the prosthesis will eventually wear out or otherwise need replacing. Body joint prostheses typically wear out faster than other prostheses because of the bearing loads they receive. Such bearing loads also make it more difficult for the bone, if damaged during the replacement operation, to heal and bond as strongly with the prosthesis. During surgery to replace the worn out element, it is thus better to leave the bone undisturbed. Prostheses used to replace high load bearing joints like hid joints may need to be repaired or replaced as often as every fifteen years of use or less. Recognizing that the present invention is applicable to other body parts as well, hip joint prostheses are discussed here for the purpose of example only.
Hip joint prostheses generally have an acetabular and a femoral component. The acetabular component typically includes a cup shaped metal shell which is anchored in the patients acetabulum and a polymeric socket liner seated in the metal shell. The femoral component typically include, a hip stem which is embedded in the top of the patients femur and a metal or ceramic ball mounted on the neck of the stem. The ball and socket articulate one inside the other and are directly load bearing together. It is usually the polymeric liner that wears out the fastest and needs replacement first. Such replacement operations are typically very traumatic for the patient. If the bone-to-prosthesis bond is broken or otherwise weakened during the operation, the patient may need to remain in bed or otherwise limit their mobility, such as by being confined to a wheelchair, to eliminate or at least significantly reduce applying bearing loads to the prosthesis and thereby to the bone. Having to remain so immobilized only adds to the suffering and inconvenience endured by the patient.
Body joint prostheses have been designed so as to permit replacement of load bearing components. For example, many hip joint prostheses are modular in construction with a variable locking system that allows a worn liner to be removed from the shell and a new liner reinstalled. One chronic problem with prior modular joint prostheses is that even though they may remain interlocked, micromovement between the liner and shell still occurs during use. This micromovement has been found to generate wear particles mainly from the polymeric liner. In addition to speeding up the wear process, it has been found that sufficient quantities of these wear particles can generate a bone disease called osteolysis. Osteolysis destroys healthy bone, and since it seldom causes pain, in many cases the disorder has caused sufficient bone damage to weaken healthy bone to the point of becoming susceptible to fracturing before the patient knows anything is wrong. With prior locking systems, the liner is typically snapped in and out of the shell. While some are designed to inhibit micromovement more than others, there is still a need to further limit such micromovement in order to keep the bone healthy and lengthen the life of the joint prostheses.
By making the cud component of single piece or unitary construction (i.e., permanently fixing the shell and liner together), such micromovement and the resulting wear particles can be eliminated. Some prior hid joint prostheses have cup components with the liner permanently molded in the shell. However, with such single piece cup components, if the liner wears out, the shell must be removed from the bone. Replacing the entire cud component requires more invasive surgery (e.g., resecting the bone) than simply replacing the liner. Along with the additional pain and inconvenience associated with such invasive surgery, there is a risk that the bone will reject the new prosthesis and not bond with the new cud component.
Therefore, there is a need for a prosthesis of modular construction which behaves more like one of single piece construction. More particularly, there is a need for a bone anchored prosthesis having components that can be readily replaced without substantial trauma to the bone, and at the same time, one that is less likely to permit micromovement between its components.